Some developments on 2.4GHz video link and 868MHz datalink antenna systems:

          Measuring the 2.4GHz Antenna SWR


                         The Ground Tracking Antenna


This blog shows an integrated 868MHz xBEE datalink antenna and tranceiver and a 2.4GHz Video downlink antenna and transmitter as a single modular unit. Feed it 3.3volts and video as well as the autopilot datastream.

Also shown is a ground tracking antenna system. Two antenna are contained in the head - a multiturn 13dB gain helical antenna for the 2.4GHz video downlink, and a Diamond loop , over the ground plane, giving 8dB gain, for the 868MHz datalink. The antenna base contains a Lipo battery and fits onto a tripod. The base unit is then lined up with North, and the unit tracks the aircraft in flight using the A/C GPS position. Cable wind/unwind software is also implemented to allow the unit to unwind for example when flying circles around the GCS - this process follows a computed geometry to ensure minimum time pointed away from the A/C, and uses the aircraft flightplane to estimate the postion to slew to to start tracking again, if the link were lost.

We have just completed range tests, first with the aircraft mounted on a tall pole, on a high ridge in the desert here in Namibia. The aircraft was able to be rotated 360deg in azimuth and +- 25deg in elevation. Tests were conducted with the ground tracking antenna at 10km from the aircraft position.

Video TX power was 27dBM @ 2.432GHz, into the Skew Planar Wheel. Datalink TX was a 868MHz xBee Pro- range was tested at 25milliwat TX power ( can be set to 1mw, 25mw, 100mw, 200mw and 300mw - 1mw at 10km did not work)

Video was perfect picture with the aircraft at any azimuth or pitch angle ( combined) except for the Aircraft directly facing the ground antenna, at 0deg pitch - the A/C antenna is shielded by the avionics, the on-board IR gimballed IR camera, and the Lipo batteries. Tilting the A/C up or down 5 degrees brought the video back with perfect picture.

Datalink comms was constant with NO dropouts at 25mw bothe ends, regardless of the A/C attitude and azimuth heading. The aircraft was also fitted with 2.4meter carbon fibre covered wings - 315mm chord, and a carbon V tail ( very similar to my mk-I SurVoyeur A/C , only greater wings span and carbon wings.- see the SurVoyeur blogs..)

The Datalink antenna consists of two rings, spaced apart, and is a Slot Antenna, ie, the radiation takes place from the slot formed between the two rings. It is a narrow band antenna- not more than 20MHz bandwith for 1.3:1 SWR - at center frequency it is 1.05:1 SWR.  The first rings were done in copper wire to determine the antenna factors, and then a PCB version made on the router, at various sizes, to determine the size reduction factors due to the PCB dielectric constant.



Wire Rings Versus PCB Versions

An Integrated model was then machined on the router - this module consists of the antena rings and is fitted with the xBee datalink module and serial interface electronics.




This module is later fitted with the Skew Planar Wheel 2.4GHz antenna, and with the video TX module.

The Skew Planar Wheel was constructed thus:

Pre-Cut element wires:                                                                       Wires formed in a 'stretch' jig.




                               Wires Stretched and removed from jig


Wires Formed.


                              Coax End pieces about to be fitted


Coax End Pieces soldered to coax outer and inner



                Antenna elements fitted to the mounting Jig and soldered to the coax ends




The 2.4GHz antenna is then fitted to the 868MHz module:


The Video Transmitter is fitted on the underside:


This assembly is then fitted into the antenna bay of the composite fuselage:


SWR measurements were taken in-situ to determine any de-tuning:


SWR AT 2.44GHz:: = 1.13:1


The ground tracking antenna basics follow:

This Consists of a base unit containing the Lipo Battery, the drive electronics, an azimuth servo drive and feedback mechanical system and a yoke carrying the antenna head. The antenna head contains the 2.4GHz helical antenna, the 868MHz datalink Diamond loop antenna, the Video receiver, the xBee datalink tranceiver, and the elevation servo with position feedback.

The Head unit closed:                                                                   The base unit guts:



Azimuth Mechanics and feedback POT


                 The Helical and Diamond antenna in the head unit.



We then flew a max range test to see what a typicall max range might be - Video was lost at 39km, and datalink was still error free with 200milliwats at that range...Video was 'snow free' ( P4 signal) at 32km.


The Nampilot....

E-mail me when people leave their comments –

You need to be a member of diydrones to add comments!

Join diydrones


  • Hi Martin,

    The Diamond basically a full wavelength loop spaced in front of a reflector, giving very useful gain. There is no interference since the video is on 2.4GHz and the datalink on 868MHz. They are not harmonically related.

    The setup works very well.

    If you are interested a useful pdf is in the link:


  • Could you elaborate on the choice of antennas? I can understand the circularly polarized antennas for video, but the slot and diamond antennas for telemetry. Never seen a diamond loop antenna before. Is it a cousin of the quad antenna?

    The setup itself looks neat, but I'm wondering how you have avoided interference issues with the antennas and radios placed so close together.

  • How about a 433Mhz telemetry and a 1.2Ghz Video combo. Can it be done?


  • Great thanks for the insight - less is more!
  • Hi Paul,

    Since we wrote all the autopilot and GCS code, and since the system is totally autonomous during flight, we have data between A/C and ground only to give A/C position at typically every 5 10 seconds, if in basic flight ( with some other basic info - height, speed, batter, etc, but a very short message. Waypoint acquisition is sent when acquired  etc, so data rates are very low. When we are controlling a camera from ground, such as the IR camera gimbal, the rates increase from ground up, but in a 30minute flight, with 20minutes of camera manipulation, we only use 2.4minutes out of 30 for data transmit time, nicely below the 6minutes/60minute duty cycle. We are able to command a modem reset in an 'emergency' but steer  away from that as we want very much to remain fully within the legal ruling...

  • Really impressive work - I love the combined transmitt module, lovely work.

    When I looked at the 868Mhz xbees a long time ago they alway had a duty cycle issue - is that gone or do you work around it/ live with it

    Cheers Paul
  • Great guys keep it up.

This reply was deleted.